According to Rashi on Exodus 33:11, משה MOŜE ascended HarSinai on 19 Tammuz to pray for the people.

י״ט בתמוז ב׳תמ״ח19th of Tammuz, AM 2448

הר סיני Har Sinai · W. Asia

https://thisjewish.day/article/2448-04-19-mo%C5%9De-ascended-har-sinai